Symbol displaying electron discharge tubes unsing secondary emission



4 I. R. SINCLAIR SYMBOL DISPLAYING ELECTRON DISCHARGE TUBES USING SECONDARY EMISSION Flled May 15, 1961 INVENTOR ATTORNEYS United States Patent 3,144,578 SYMBGL DHSPLAYING ELECTRON DISCHARGE TUBES Uh lG SECONDARY EMlSSlOlfJ Ian Robertson Sinclair, Chelnisford, England, assignor to English Electric Valve Company Limited, London, England, a British company Filed May 15, 1961, Ser. No. 110,250 Claims priority, application Great Britain May 16, 1960 3 Claims. (Cl. 31512) This invention relates to symbol displaying electron discharge tubes and has for its object to provide improved, simple, and reliable electron discharge tubes capable of displaying, at will, any of a number of pre-determined symbol-cg, letters or numerals-visually.

According to this invention a symbol displaying electron discharge tube comprises within an evacuated envelope, a fluorescent screen; an electron gun adapted to provide an electron beam to flood said screen substantially uniformly with electrons; and, in the path of the beam to said screen, two spaced mesh electrodes extending transversely across said path and having between them a plurality of transversely arranged symbol electrodes, insulated from one another and each having its own connection electrode leading to the outside of the envelope, each symbol electrode being shaped in accordance with a desired symbol to be displayed and each being provided on the side thereof facing the gun with a layer of insulating material having a relatively high co-efiicient of secondary emission and a relatively low first secondary emission cross-over.

There are many insulating materials which are suitable for use as layers on the symbol electrodes of a tube in accordance with this invention, examples of preferred materials being magnesium fluoride, magnesium oxide and tale.

The symbol electrodes may conveniently be made of wires bent into the required shapes, e.g., the shapes of letters or numerals.

In a preferred construction a tube comprises an electron gun having a control grid electrode and positioned at or near one end of the envelope; an accelerator electrode adjacent said gun; a fluorescent screen at or near the other end of the envelope; two spaced mesh electrodes which are electrically connected together and are substantially parallel to said screen; a plurality of symbol electrodes between said mesh electrodes and substantially parallel thereto, each having a layer of insulating material on the side theroef towards the gun; and, between the accelerator electrode and the mesh electrode nearer thereto, a collimator electrode adapted to collimate into substantially parallel paths, the electrons of a beam of electrons projected from said gun past the mesh and symbol electrodes to the screen.

In use the cathode, control grid and collimator electrodes are operated at such potentials in relation to one another and to the potentials applied to the other electrodes of the tube as to collimate the beam before it reaches the first mesh electrode in its path; the mesh electrodes are operated at a potential which is positive in relation to the gun cathode; the symbol electrodes are operated (when it is not desired to display any symbol) at a potential a little below the first crossover potential of the secondary emitting insulating material thereon; and the fluorescent screen is operated at a potential which is not negative with respect to the mesh electrode potential and is pie ferably substantially positive with respect thereto. If, with the applied potentials as just described, the gun cathode is pulsed negatively, the electrons of the beam will strike the layers of secondary emitting material on the symbol electrodes at such velocity as to cause them to 3,li4,578 Patented Aug. 11, 1964 charge up to the potential of the nearest secondary electron collector electrode, i.e., a mesh electrode. In this condition the symbol electrodes cause minimum interference with the beam and the screen will be substantially uniformly illuminated. If now, one particular symbol electrode is brought to a sufiicient negative potentialas, for example, by applying a pulse of sufllciently negative potential to the selected symbol electrode-so that it is driven to a potential well below that of the first secondary emission cross-over of the insulating layer material thereon, the said layer will assume the gun cathode potential by process of cathode potential stabilisation of an insulator. In this condition the electrons of the beam will be repelled from the region of the particular symbol electrode chosen and that symbol will be displayed as a shadow on the screen. Any symbol thus displayed can be erased by pulsing the cathode negatively and another symbol can then be chosen for display.

The invention is illustrated in the accompanying drawing which shows diagrammatically one embodiment.

Referring to the drawing, the tube therein shown has a bulbous glass envelope 1 with an electron gun having a cathode 2 and a control grid electrode 3 in the neck end. On or near the large transparent end wall of the envelope is an ordinary fluorescent or phosphor screen 4. The external connection to the screen is shown at 5. Parallel to one another and to the screen are two mesh electrodes 6 and 7 extending right across the tube and connected electrically to one another and to an external connection 8. Between the meshes 6 and 7 is a plurality of symbol electrodes 9 shaped in accordance with desired different symbols and each made of wire with a coating of insulating material, such for example, as magnesium fluoride, on the side thereof towards the mesh 6 and the gun. The symbol electrodes are parallel to one another and to the meshes 6 and 7, each has its own connection to the outside of the envelope, and they are as close together as is consistent with the maintenance of proper insulation between them. In a practical case the separation of the meshes 6 and 7 is about /2" and each may have an electron transmission factor of about 60%. So as not to complicate the drawing, the symbol electrodes are shown merely as lines of different lengths and an external connection, marked it), is shown to only one of them.

Adjacent the gun is an accelerator electrode 11 and between this and the first mesh 6 is a collimator electrode 12.

In use the cathode 2 may be at earth potential or the control electrode 3 at a negative potential sufiicient to e11- sure collimation of the beam before entry to the mesh 6 if the other operating potentials are suitably chosen. The meshes 5 and 7 may be operated at, say, vol-ts and the symbol electrodes at a normal potential (lie, when no symbol is required to be displayed) just below the first secondary emission cross-over potential of the insulating material thereon which potential may, in a practical case, be about 50 volts. The operating potential of the screen may be about 500 volts.

I claim:

1. A symbol displaying electron discharge tube comprising within an evacuated envelope, a fluorescent screen; an electron gun adapted to provide an electron beam to flood said screen substantially uniformly with electrons; and, in the path of the beam projected to said screen, two spaced apart mesh electrodes extending transversely across said path and a plurality of transversely arranged symbol electrodes disposed between said mesh electrodes, said symbol electrodes being insulated from one another and each having its own connection electrode leading to the outside of the envelope, each symbol electrode being shaped in accordance with a desired symbol to be displayed and each being provided on the side thereof facing the gun with a layer of insulating material having a relatively high coeflicient of secondary emission and a relatively low first secondary emission cross-over potential.

2. A symbol displaying electron discharge tube comprising within an evacuated envelope, an electron gun positoned adjacent one end of said envelope for projecting a beam of electrons, said gun including a control grid electrode; an accelerator electrode adjacent said gun; a fluorescent screen adjacent the other end of said envelope; two spaced apart mesh electrodes mounted substantially parallel to said screen and means electrically connecting said mesh electrodes together; a plurality of symbol electrodes disposed between said mesh electrodes and substantially parallel thereto, each symbol electrode having a layer of insulating material on the side thereof towards said gun, the insulating material having a relatively high coeflicient of secondary emission and a relatively low first secondary emission cross-over potential; and a collimator electrode for collimating said electrons into substantially parallel paths extending past said mesh electrodes and said symbol electrodes to the screen, said collimator electrode being disposed between said accelerator electrode and the mesh electrode nearer thereto.

3. A symbol displaying electron discharge tube comprising within an evacuated envelope, a fluorescent screen; an electron gun projecting an electron beam toward said screen to flood the screen substantially uniformly with electrons, said gun including a control grid electrode and a cathode; first and second spaced apart mesh electrodes positioned to intersect the projected electron beam; a plurality of transversely arranged symbol electrodes disposed between said mesh electrodes, said symbol electrodes being insulated from one another and each having its own connection electrode leading to the outside of the envelope, each symbol electrode being shaped in accordance with a desired symbol to be displayed and each being provided on the side thereof facing said gun with a layer of insulating material having a relatively high coefiicient of secondary emission and a relatively low first secondary emission cross-over potential; 21 collimator electrode for colli-mating the electrons of said beam into substantially parallel paths; means for applying to said cathode, said control grid electrode, and said collimator electrode operating potentials which collimate said beam before it reaches said first mesh electrode; means for applying to both mesh electrodes a potential which is positive with respect to the gun cathode potential; means connected to said connection electrodes for applying to those symbol electrodes that are not to be displayed at any given time a potential slightly below said cross-over potential; means also connected to said connection electrodes for applying to any selected symbol electrode to be displayed at any given time a pulse of negative potential sufiicient to drive the selected symbol electrode Well below said cross-over potential; means for applying to said screen a positive potential at least equal to that of the mesh electrodes; and means for applying pulses of negative potential to said gun cathode.

References Cited in the file of this patent UNITED STATES PATENTS 2,927,237 Lieb Mar. 1, 1960 2,964,672 Nixon Dec. 13, 1960 3,005,922 Hart Oct. 24, 1961 

1. A SYMBOL DISPLAYING ELECTRON DISCHARGE TUBE COMPRISING WITHIN AN EVACUATED ENVELOPE, A FLUORESCENT SCREEN; AN ELECTRON GUN ADAPTED TO PROVIDE AN ELECTRON BEAM TO FLOOD SAID SCREEN SUBSTANTIALLY UNIFORMLY WITH ELECTRONS; AND, IN THE PATH OF THE BEAM PROJECTED TO SAID SCREEN, TWO SPACED APART MESH ELECTRODES EXTENDING TRANSVERSELY ACROSS SAID PATH AND A PLURALITY OF TRANSVERSELY ARRANGED SYMBOL ELECTRODES DISPOSED BETWEEN SAID MESH ELECTRODES, SAID SYMBOL ELECTRODES BEING INSULATED FROM ONE ANOTHER AND EACH HAVING ITS OWN CONNECTION ELECTRODE LEADING TO THE OUTSIDE OF THE ENVELOPE, EACH SYMBOL ELECTRODE BEING SHAPED IN ACCORDANCE WITH A DESIRED SYMBOL TO BE DISPLAYED AND EACH BEING PROVIDED ON THE SIDE THEREOF FACING THE GUN WITH A LAYER OF INSULATING MATERIAL HAVING A RELATIVELY HIGH COEFFICIENT OF SECONDARY EMISSION AND A RELATIVELY LOW FIRST SECONDARY EMISSION CROSS-OVER POTENTIAL. 